From a theoretical view, at least four basic schemata commonly used in considerations of inflation can be distinguished.

The quantity theory

The first of these and the oldest is the view that the level of prices is determined by the quantity of money. The ratio of the stock of money that people want to hold to the value of the transactions they perform each year (or the inverse of this ratio, called the velocity of circulation) is supposed, in the simplest version of this view, to be fixed by such factors as the frequency of wage payments, the structure of the economy, and saving and shopping habits. So long as these remain constant, the price level will be directly proportional to the supply of money and inversely proportional to the physical volume of production. This is the celebrated quantity theory, going back at least as far as David Hume in the 18th century. But the theory assumes that productive capacity is fully employed, or nearly so. Because, in fact, the extent to which productive capacity is used varies a great deal—indeed, sometimes more than the level of prices—the quantity theory fell into disfavour between World Wars I and II, when the level of activity provided more reasons for anxiety than did the long-run movement of prices.

In a refined version, the quantity theory was revived by Milton Friedman and other University of Chicago economists in the 1950s and ’60s. Their basic contentions were that short-period changes of the money supply are, in fact, followed (after a varying interval) by changes in money income and that the velocity of circulation, though it fluctuates to some extent with the money supply, tends to be fairly stable, especially over long periods. From this, they concluded that the money supply, while not a reliable instrument for controlling short-term movements in the economy, can be effective in controlling longer term movements of the price level and that the prescription for stable prices is to increase the money supply regularly at a rate equal to that at which the economy is estimated to be expanding.

Against this, it has been argued that in highly developed economies the supply of money varies largely with the demand for it and that the authorities have little power to vary the supply through purely monetary controls. The correlations observed by this so-called Chicago school between money supply and money income are attributed by their critics to variations in the demand for money to spend, which elicit partial responses from supply and are followed after an interval by corresponding changes in money income. The relative stability of the velocity of circulation is attributed by them to the facility with which the supply of money accommodates itself to demand; they argue that insofar as supply may be restricted in the face of rising demand, velocity will increase, or (what really amounts to the same thing) new sources of credit, such as trade credit, will be exploited.

The Keynesian theory

The second basic approach is represented by John Maynard Keynes’s theory of income determination. The key to it is the assumption that consumers tend to spend a fixed proportion of any increases they receive in their incomes. For any level of national income, therefore, there is a gap of a predictable size between income and consumption expenditure, and to establish and maintain that level of national income it is only necessary to fix expenditure on all nonconsumption goods and services at such a level as to fill the gap. Apart from government outlays, the main constituent of this nonconsumption expenditure is private investment. Keynes supposed investment to be fairly sensitive to the rate of interest. The latter, in turn, he supposed to be negatively related, up to a point, to the stocks of “idle” money in existence—in effect, positively related to the velocity of circulation of money. He held, moreover, that there is a floor below which long-term interest rates will not fall, however low the velocity of circulation. These relationships between interest and idle money (or the velocity of circulation) have been pretty well supported empirically.

John Maynard Keynes, detail of a watercolour by Gwen Raverat, about 1908; in the National Portrait …

The chief importance of the Keynesian approach and various elaborations of it is that they provide a framework in which governments can endeavour to manage the level of activity in the economy by varying their own expenditures and receipts or by influencing the level of private investment. This has been a principal basis of policy in many industrialized countries in recent decades. Difficulties in practice have sprung from uncertainty about, or changes in, the underlying quantitative relationships and the existence of uncertain time lags in their operation, which make it hard to deal effectively with unforeseen contingencies. The uncertainty and weakness of the relation between interest rates and private investment are another source of difficulty. Many economists believe, however, that the approach has led to better control over short-term changes in employment and real income.

In the form in which it has just been stated, however, the Keynesian approach does not offer much insight into movements of the price level. The simplest variant of it that will do so is based on the view that inflation arises entirely from attempts to buy more goods and services than can be supplied—i.e., more than can be produced at the “full employment” level of activity. If, for example, government expenditure is higher than the difference between production and consumption at the level corresponding to full employment, there is an “inflationary gap.” The market process closes this gap by a bidding up of prices to the point at which the difference between income and consumption, in money terms, is big enough to accommodate the government expenditure. (In an economy open to foreign trade, the gap may be closed wholly or in part by the creation of an import surplus). The theory fails to account for the experience in the decades after World War II of continuous inflation in conditions that do not suggest the existence of an inflationary gap.

A third approach in the analysis of inflation assumes that prices of goods are basically determined by their costs, whereas supplies of money are responsive to demand. In these circumstances, increasing costs may create an inflationary pressure that becomes continuous through the operation of the “price-wage spiral.” The supposition is that wage earners and profit receivers (neglecting for the moment other groups in the economy) aspire to incomes that add up to more than the total value of their production at full employment. One or both groups must, therefore, be dissatisfied at any given time. The wage earners, if dissatisfied, demand wage increases. These are conceded (at least in part) by employers in the course of the bargaining process, initially at the expense of profits. Later, employers increase prices to reflect their higher costs, and, while this restores profits, it also reduces wage earners’ real incomes, sowing the seeds of a further round of wage demands. If the supply of money were fixed, this process would lead to increasing monetary stringency; it would become increasingly difficult to finance increases in wages and purchases of goods the prices of which had just been raised or, indeed, to finance production and distribution generally—though, as noted earlier, there are some circumstances in which the velocity of circulation can rise drastically and make a limited money stock go a long way. In practice, money supply responds to demand, partly because monetary authorities do not wish to see the dislocation of capital markets that would follow if monetary stringency produced very large rises in rates of interest.

In the 1960s there was much discussion of a relation named after the British economist A.W. Phillips (though in a rudimentary form it can be traced to earlier writers), whereby the rate of increase of wages was shown to vary negatively with the level of unemployment. This can be interpreted as signifying that the price-wage spiral proceeds more rapidly at high levels of economic activity than at low levels. The empirical evidence for the “Phillips curve” was not entirely satisfactory, and the hopes that had been excited in some quarters that a higher but still politically tolerable level of unemployment would reduce or end inflation were shaken by the rapid wage inflation that occurred during the severe recessions in later decades.

The structural theory

The fourth basic approach to the inflationary process is not entirely independent of some of those just discussed; its distinguishing feature is its emphasis on structural maladjustment in the economy. One version of it depends upon the simple proposition that resistance to reductions of money wages is so strong that they hardly ever take place. If this is so, then all adjustments of wages to take account of relative changes in the supply of, and demand for, labour in different industries or occupations have to be accomplished through the absolute raising of all wages except those of the group of workers whose market position is weakest. The rate of wage inflation as a whole is then seen as proportional to the rate of structural change in the economy.

Another version, held to be appropriate to some developing countries, focuses on the gap between imports and exports. Imports tend to increase faster in those countries (because of the rising demand for manufactured goods) than the ability of the traditional exporting industries to pay for them. Difficulty is experienced in substituting home manufactures for imports, partly because home markets are often too small to support the required industries and partly because the development of manufacturing itself requires extensive imports of machinery and structural materials. Consequently, there is a continuous downward pressure on the international value of the country’s currency; this is felt in a continuous upward pressure on the country’s internal prices.

Alternatively, inflation in such countries may result from social and political pressures to provide employment for the overflow into the towns of a rapidly growing rural population; since there is a shortage of savings, this leads to excessive creation of new credit in one way or another and thus to a straightforward “demand-pull” inflation. The chronic inflationary tendencies in some Latin American countries have been attributed to mechanisms of these kinds.